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A215713
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Denominator of sum(i=1..n, 3*i/4^i).
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2
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4, 8, 64, 16, 1024, 2048, 16384, 16384, 262144, 524288, 4194304, 2097152, 67108864, 134217728, 1073741824, 1073741824, 17179869184, 34359738368, 274877906944, 17179869184, 4398046511104, 8796093022208, 70368744177664, 70368744177664, 1125899906842624
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OFFSET
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1,1
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COMMENTS
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The odd-indexed terms are the even-indexed powers of 4 (A013709).
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REFERENCES
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Calvin C. Clawson, The Beauty and Magic of Numbers. New York: Plenum Press (1996): 96.
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LINKS
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Colin Barker, Table of n, a(n) for n = 1..1000
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EXAMPLE
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a(4) = 16 because 3/4 + 6/16 + 9/64 + 12/256 = 3/4 + 3/8 + 9/64 + 3/64 = 48/64 + 24/64 + 9/64 + 3/64 = 84/64 = 21/16.
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MATHEMATICA
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Table[Denominator[Sum[3i/4^i, {i, n}]], {n, 40}]
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PROG
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(MAGMA) [Denominator(&+[3*i/4^i: i in [1..n]]): n in [1..25]]; // Bruno Berselli, Sep 03 2012
(PARI) vector(100, n, denominator(sum(i=1, n, 3*i/4^i))) \\ Colin Barker, Nov 09 2014
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CROSSREFS
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Cf. A215712 for the numerators. A036295/A036296 is very similar but with i/2^i instead of 3i/4^i. Cf. also A122553.
Sequence in context: A303284 A275574 A214590 * A120777 A091095 A075787
Adjacent sequences: A215710 A215711 A215712 * A215714 A215715 A215716
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KEYWORD
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nonn,easy,frac
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AUTHOR
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Alonso del Arte, Aug 21 2012
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STATUS
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approved
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